German Offenlegungsschrift No. 33 09 386 discloses a bellows, the clamping sections of which are configured by procedures during their production so that they have an elasticity which the thermoplastic material itself does not possess. This elasticity is intended to keep the clamping sections airtight over a longer period of time and to provide for a constant firm seat.
In recent times those parts of bellows which do not include the clamping sections are most often made of a thermoplastic elastomer material. This material is inexpensive to work and satisfies most of the requirements, especially in the automobile industry, with respect to mechanical properties and chemical stability against mineral oils and moisture.
On the other hand, a disadvantage of this material is that it has about twice a high a compression set as elastomer mixtures. Also, there is the problem of cold flow, which is a negative property especially for the clamping sections of bellows.
In an attempt to remedy this problem, self-tightening band clamps are used, or a rubber base is cemented under the band clamp. This special configuration of the clamping sections, however, adds significantly to the cost of manufacture of the bellows.
Furthermore, when the band clamps are tightened, the pressure around the entire circumference of the clamping section is not uniformly distributed, and pressure peaks are produced which lead to premature destruction of the thermoplastic elastomer material.
When thermoplastic elastomers are used in the manufacture of bellows, it often happens, especially when thermal stress is involved, that the material "squeezes out" from under the clamps at both ends of the bellows, so that the clamp can become loose. This phenomenon in conjunction with thermoplastic materials is known as "cold flow."